codes like below:

Public Class Form1

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Dim n As Integer = 3
Dim b As Integer = 7
Dim c As integer
Dim k As String
Dim x As Integer

If n > 1 Then
If x = 5 Then
k = "23"
End If

If x = 6 Then
k = "33"
End If
Else
k = "99"
End If

If b > 5 Then
c = b + k
Else
c = b - k
End If
textbox1.text = c
End Sub


when I run the codes above, error message indicated:
Variable k is used before it has been assigned a value.A null reference exception could result at runtime

pls,thanks